If anyone is interested.

I found an app called Orux Maps. On Android Play Store it's £3.something, if you do some surfing the .apk is available for free, it did what I needed so I paid for it.

Using the Orux Maps app I've got the Open MTB maps on my phone, tablet and PC all showing the same thing and all working offline. I'm in Spain at the moment so this is sort of a big deal for me. I can plot routes on the tablet, save as a .gpx and move to my Garmin.

The reason for the post is that I'm living in my van, power and data are key elements. The battery on my tablet lasts a lot longer than the laptop so I don't need to switch the inverter on and use more power. Anything I can do offline saves me paying for data.

It's a convoluted way of boasting about being in Spain but I'll allow it.

Will have a look at that app, cheers. Haven't fully deceided what to do wrt to the death of Viewranger.

+1 for Orux Maps. I've used it for years, and it really is excellent. You can load tonnes of different map formats into it (online and offline) and it allows you to download online maps to save on your device from the app itself.

Topo 4umaps in the "World" section of online maps is a good OpenStreetMap render focused on outdoor use that you can immediately use. As Pirahna says, you can load in pre-rendered, or raw OSM data from places like Open MTB Map and apply custom Mapnik rendering to show stuff such as terrain quality, technicality, etc etc

Always surprises me no company producing Ordnance Survey map based websites have teamed up with the Orux Maps team to have Orux as the mobile companion app for their webapp.